Dry-process developing devices using a powdery developing agent have widely been employed in image forming apparatuses such as electronic copiers, printers and facsimiles, in which a latent electrostatic image formed on a latent image bearing member is visualized with a developer to obtain a recorded image.
In recent years, color image forming apparatuses using electrophotographic process have broadly been employed, and digitized images are easily available. Thus, it is required to make an image to be printed at higher definition. While studying higher resolution and gradation of an image, as an improvement of a toner which visualizes a latent image, it has been studied to further conglobate and minimize in particle size for forming the image at high definition. And, since in the toners produced by the pulverizing methods, their conglobation and minimization are limited, so-called polymerized toners produced by a suspension polymerization method, an emulsification polymerization method and a dispersion polymerization method capable of conglobtaining and minimizing in particle size have been being employed.
The polymerized toner has a small particle diameter and thus has high adhesion force to members, leading to degradation in transfer efficiency and occurrence of filming. The polymerized toner also has a spherical shape and thus has poor cleanability. In addition, in the process of producing the polymerized toner, toner components having lower resistance are localized in the vicinity of the surfaces of the toner base particles, so that the chargeability becomes lower to cause background smear.
Furthermore, since there is a high need for a toner having low-temperature fixing property to achieve energy saving, it is desired to use a binder resin having a low melting temperature. However, the toner having low-temperature fixing property has a newly arising problem relating to heat resistance storageability.
In view of this, attempts to solve these problems by modifying surfaces of toner base particles have been made. The method for surface modification disclosed is, for example, a method in which surfaces of toner base particles each containing a first resin particle and a colorant are partially or entirely coated with second resin particles (see, for example, PTL 1). However, in this disclosed method, the second resin particles are too varied and ununiform. The obtained toner particles increase in cleaning property but not sufficiently improve in background smear nor storageability. In addition, they also cause degradation in transferability.